What is Sildenafil Citrate 100mg? What is the difference between sildenafil and Levitra and Cialis? All three come from the same class of medicine (PDE5 inhibitors) and work in the same way. They are all equally effective but only have different durations of action. Sildenafil Citrate 100mg stays active for about 4 hours, Levitra for 4-5 hours…